1. Field of the Invention
The present invention relates to printing paper, more particularly to a plain paper type of printing paper useful both for ink-jet printing and for electrophotographic printing without special coating on the printing face.
The present invention also relates to a method of forming an image employing the above printing paper.
2. Related Background Art
The ink-jet printing system, which ejects ink directly onto printing paper, is attracting attention owing to low running cost, less noise, and ease of color printing in comparison with other conventional printing systems.
In an ink-jet printing system, an aqueous ink is employed in ink-jet printing system in view of safety in handling, and printing characteristics. The printing medium for ink-jet printing is required to absorb the ink rapidly, not to cause mixing of superposed different colors of inks, to allow the ink spread appropriately, to allow the ink to form ink print dots in a nearly true circle shape with sharp dot edges at high density, and naturally to be sufficiently white to exhibit contrast of dots.
On the other hand, in electrophotographic printing system, the printing paper is required to have appropriate surface electric resistance and appropriate surface smoothness, to have satisfactory toner-transferring properties, to cause little paper dust, not to cause defects of images by adhesion of paper dust on a photosensitive member and an electrostatic charging roll, etc., to be attracted well to the transfer drum, and so forth.
Ink-jet printing paper for exclusive use for ink-jet printing is disclosed in JP-A-1-135682 to satisfy the above requirements for the ink-jet printing system. However, for mono-color printing and business color printing by ink-jet system, use of inexpensive usual plain paper is desired like the one generally used in electrophotographic printing.
On the other hands, in electrophotographic printing, neutralized paper has come to be used in place of conventionally used acidic paper for improvement in storability and other purposes. JP-A-51-13244, JP-A-59-162561 and JP-A-2-54543, for example, disclose neutralized paper as toner-transfer paper having excellent electrophotographic printing characteristics.
The conventional plain paper for electrophotographic printing, when used for ink-jet printing, has disadvantages such that ink absorbency is so low that ink spreads out unnecessarily if a large amount of ink is applied, and that the ink is absorbed along paper fibers to result in unsharpness of the ink dot shape.
For offsetting such disadvantages, the inventors of the present invention proposed previously a specified ink for ink-jet printing and an ink-jet printing method employing the ink. In this ink an amount of a high-boiling organic solvent which serves prevention of drying and clogging of ink-ejection nozzles is decreased, and a nitrogen compound as a dissolution aid for the dye such as ammonia, urea, and their derivatives, aminoalcohols, alkylamines, and amino acids is contained.
When toner-transfer paper, which is made of neutralized plain paper and widely used in electrophotographic printing, is used for ink-jet printing, the black ink used in the ink-jet printing tends to develop brown color on the paper, the phenomenon being called "bronzing". The above ink containing the nitrogen compound exhibits significant bronzing in ink-jet printing, disadvantageously.
The neutralized paper, when used for electrophotographic printing, has not necessarily satisfactory properties, and cannot be free from disadvantages of paper dust formation, abrasion of the photosensitive drum, fixing rolls, paper-delivery rolls, etc., and insufficient attraction of paper by the transfer drum.
Under such circumstances, with popularization of ink-jet printing system owing to its advantages, the printing paper is earnestly desired which is useful both for electrophotographic printing system and for ink-jet printing system.